Group leaders: Per Nørnberg, Haraldur Pall Gunnlaugsson

Background

"The red planet" has its name from the reddish colour by which Mars appears in the sky on clear evenings. The reason for this is that a layer of surface dust on Mars consists of iron oxides. The red colour of Mars was determined by Earth based reflection spectroscopy, and confirmed by optical studies on the Viking-missions and the Pathfinder mission in 1997 from which the following view is taken (Pathfinder view).

Mars landscape (NASA)

A number of iron oxides are known from the Earth to be ochreous or red coloured (Hematite, Maghemite, Ferrihydrite, Goethite and Lepidocrocite). Common for the reddish or yellowish iron oxides are that they contain iron in the oxidation state +III. The presence of iron oxides on the surface of Mars is no surprise as iron (Fe) is known to be the third most common element (after oxygen (O) and silicon (Si)) in the surface material of the planet.

However, the primary minerals at the surface of Mars are known to contain nearly only iron in the oxidation state +II. This is known from studies of Martian meteorites, which are assumed to be surface rocks ejected from Mars upon meteorite impacts at the planet, and also known from study of reflection spectra taken of the Martian surface. Thus, during formation of the Martian soil or surface dust oxidation of the iron from +II to +III took place. How did this happen?

Structure from ALH84 Mars meteorite (NASA)

Basically, two possibilities are discussed: "In situ" oxidation of the primary rocks by the strong UV radiation at the surface of Mars, and Weathering and dissolution of the primary rocks in a wet and oxidating atmosphere, possibly with microbiology taking part in the processes

The iron oxides at the surface of Mars have in their mineralogy built in information on their formation history, and thus information on the earlier surface environmental conditions. This is the reason why the missions to Mars are studying the iron oxides as detailed as possible.

Salten Skov Mars analogue

As samples of the red dust from Mars are not available, background studies and tests of experiments to be carried out on Mars missions have to take place on Earth by using Mars analogue samples. These are samples which as far as we know at present have properties as close to Martian surface materials as possible. Samples suited for studying the red surface dust on Mars happen to be present in nature in Denmark.

Experimental facilities

Mars tank and wind tunnel

Mars sample analogues have been used in scientific experiments related to Martian surface processes (magnets, charging, adhesion, aerodynamics, Mössbauer spec., APXS). The main facility for these experiments are the Mars wind tunnel. However, a number of other analytical instruments are necessary for characterization of the mineralogical composition of the samples used. Particle size of the dust are determined by laser diffraction (instrument principle sketch). Mineralogy is determined by x-ray diffraction (XRD) and Mössbauer spectroscopy (MS) and morphology and chemical composition by scanning and transmission electron microscopy, both with EDX analyzers (energy dispersive x-ray analysis), and total chemical analyses of bulk samples by x-ray fluorescence (XRF).

Martian soil analogue

Salten Mars analogue

In mid Jutland red soils which are nearly pure precipitates of iron oxides are found at a number of places. The mineralogy of the of the soils is goethite (unmagnetic), hematite (slightly magnetic) and maghemite (strongly magnetic) (XRD-spectra). The near UV spectrum of the soil is compared to spectra from the surface material of Mars.

Magnet array

No scientific reliable explanation of the presence of the two latter, subtropical/tropical iron oxides, has so far been found. However, the magnetic phases of the soils make them suitable as Mars sample analogues. One of them (Salten Skov) proved to be captured on the magnetic array which was sent to Mars on the Pathfinder mission in 1997 in the same way as was seen with the dust on Mars.

This soil has together with laboratory produced pure samples of hematite, and maghemite been used in the magnetic capture, adhesion, charging and other experiments in the Mars wind tunnel.

Iron oxides

Magnetite Fe₃O₄ [ Fe^III (Fe^II , Fe^III) O₄ ]

Magnetite is a strongly magnetic iron oxide, which deviates from most other iron oxides by containing both divalent and trivalent iron. Magnetite is often found in beach sand as small black magnetic grains.

Maghemite g - Fe₂O₃

Maghemite is also strongly magnetic and has a structure like magnetite. It deviates from magnetite because almost all the iron is trivalent. Empty places in the structure compensate for the iron in oxidation state +III. Maghemite is known from burned sites and is very strongly red-brownish.

Hematite a - Fe₂O₃

Hematite is only very weakly magnetic. The grains are fine, almost blood-red (Greek: haima = blood). It is also known as bloodstone which is used for jewellery and is nearly black. Hematite is utilized as a dye (e.g. Swedish red).

Goethite a - FeOOH

Goethite is almost unmagnetic. The colour is yellow to yellow-orange and is also used as a dye. Goethite is a very common mineral in Danish soils.

Titanomagnetite (ulvöspinel) Fe₂TiO₄ [ Fe^II (Fe^II , Ti^IV) O₄ ]

Ulvöspinel is different from magnetite in not being a strong magnetic mineral. It has like magnetite a cubic structure and transitions between the two are common. The mineral most likely constitute part of the vulcanic rocks on Mars.